1. Field of the Invention
The present invention relates to a substrate for an SAW device, a method of manufacturing the substrate, and to the SAW device.
2. Description of the Background Art
In a mobile telephone, an electronic component referred to as an SAW filter for transmitting/receiving electric signals of only the desired frequencies is incorporated. The SAW (Surface Acoustic Wave) filter means a surface wave filter. In an SAW device such as the SAW filter, a piezoelectric substrate formed of a material having piezoelectric effect is used. Generally, an SAW device is used bonded on a holding substrate having superior radiation performance, to radiate heat generated by the piezoelectric substrate at the time of use.
By way of example, Japanese Patent Laying-Open No. 2008-301066 (Patent Document 1) discloses a composite substrate including a holding substrate having low coefficient of thermal expansion and a piezoelectric substrate of an SAW device bonded to each other.
A method of bonding the piezoelectric substrate and the holding substrate is disclosed, for example, in Japanese Patent Laying-Open No. 2004-343359 (Patent Document 2). Specifically, a piezoelectric substrate and a holding substrate are washed to remove impurities on bonding surfaces of the two substrates and, thereafter, the bonding surface or surfaces are irradiated with plasma, neutralized beam or ion beam of oxygen or inert gas, whereby remaining impurities are removed and the surface layer of the bonding surface or surfaces is activated. With the activated bonding surface or surfaces, the piezoelectric substrate and the holding substrate are bonded.
A piezoelectric substrate deforms as it receives stress from input electric signals. Therefore, the holding substrate mounting the piezoelectric substrate must have high strength. For this reason, some of conventionally used holding substrates for mounting piezoelectric substrate for the SAW device are formed of sapphire, as described, for example, in FUJITSU SAW filter (Non-Patent Document 1).
As the holding substrates disclosed in each of the documents cited above, mainly, sapphire single crystal substrates are used. Single crystal of sapphire, however, is typically very expensive. This leads to high-cost of production of substrates for mounting piezoelectric substrates for the SAW device.
Though sapphire has sufficient strength as a substrate for mounting a piezoelectric substrate for the SAW device, it has very high hardness and, therefore, formed substrate is sometimes prone to damage such as chipping. Further, due to the high hardness, cutting work of sapphire to a substrate of a desired shape is difficult. Consequently, speed of cutting cannot be increased, further leading to higher cost of sapphire substrates. Further, since sapphire has cleavage characteristic particular to single crystal, it is highly possible that the sapphire holding substrate split due to stress applied by the deformation of piezoelectric substrate.
In Patent Document 1, for example, the holding substrate and the piezoelectric substrate are bonded by an adhesive. For highly accurate bonding of piezoelectric substrate to the holding substrate, however, bonding utilizing van der Waals interaction between the two substrates is preferred.
When substrates are bonded using van der Waals interaction, it is preferred that bonding surfaces have superior flatness. Therefore, it is preferred that the bonding surface of holding substrate to the piezoelectric substrate is subjected to three types of polishing including rough polishing, normal polishing and polishing with diamond abrasive grains. However, even after the bonding surface of holding substrate is polished with diamond abrasive grains, when it is to be joined to the bonding surface of piezoelectric substrate, many voids tend to generate between the two bonding surfaces, resulting in failure of bonding.
According to Patent Document 2, the bonding surface is irradiated with ion beams or plasma, so that the bonding surface is activated and an amorphous layer is formed, and then, the bonding surfaces are joined. Patent Document 2, however, is silent about the polishing process of bonding surface. Therefore, even if the bonding method disclosed in this document is used, there is still a possibility of bonding failure, resulting from roughness or difference in level of the bonding surface.